It is often necessary to expose an image from a transparency onto a light sensitive receiving element. Such receiving element can be a printed wiring board, a conducting foil, elements used in etching, etc. In practice, such light sensitive receiving elements are brought into contact with the image bearing transparency in a printing frame or similar device and are exposed to a special light source. Printed wiring boards are usually preworked with an arrangement of drilled holes. However, difficulty arises in assuring good registration, i.e., an accurate fit, between the transparency and the board, especially when several boards are consecutively exposed.
To assure good registration between the transparency and the printed wiring board (hereinafter referred to as PWB), the practice has been to visually align the transparency (commonly referred to in the trade as photomask or phototool) with the PWB and secure it to the PWB with adhesive tape. This method, however, is not very accurate because it is somewhat dependent upon an operator's skill to perfectly align the phototool and PWB. Furthermore, this method is cumbersome, and its productivity is low.
In an effort to eliminate the drawbacks of such method, pins have been used through a prearrangement of holes in the phototool and PWB to align both together. At least in principle, the desired reproducibility of alignment, when several PWBs are to be exposed, can be obtained with the use of alignment pins, provided, of course, that the alignment holes are accurately located in both the PWB and the phototool to assure correct registration of the pattern to be reproduced. However, once punched, the phototool can no longer be adjusted relative to the PWB, and, therefore, any misalignment between the PWB and the phototool, usually caused by inaccurate punching of holes or by manufacturing tolerances, cannot be corrected. This disadvantage results in a high number of rejections or increased refinishing operations.
In addition to problems of alignment of the photomask, care must be taken that the mask is not stretched or marked by the means used to apply the mask to the printed circuit board.
Computer grade PC boards are typically manufactured in panel form in sizes of the order of 18 by 20 inches (0.46 M by 0.5 M). Conductor lines and spaces are of the order of 0.010 inch wide (0.025 cm) with sharply defined edges, free of nicks and bulges. Additionally, the imaging resist forming the conductors must be maintained at a constant thickness, consistent with plating or etching chemicals, temperature and immersion time. Too thin a resist results in breakdowns and plating of metal at unwanted locations.
With regard to image placement on the copper-clad board, the 18 by 20 inches (0.46 M by 0.5 M) panel will typically have an accuracy of 0.002 inch (0.05 mm) on drilled hole locations, requiring that the imaging be accurate to within 0.005 inch (0.13 mm) in order to maintain an annular ring of the order of 0.005 inch (0.13 mm) around the hole.
The UV curable photopolymers used in PWB manufacture have been developed for application by screen printing over a copper surface and cured by conveying under mercury vapor lamps. Surface temperature rise is significant, because the board receives approximately 200 watt-seconds of energy per square inch of area for polymer whose surface is exposed to air. Typically, surface temperatures in excess of 300 degrees F. are experienced. However, most of the tested photopolymers are affected by air to the extent that the exposure energy can be reduced to only 50 watt-seconds per square inch when the air is completely excluded by the mating process of the present invention.
In one method for producing graphic images on substrates, photopolymers, characterized by a composition of 100 percent reactive polymers having a paste-like consistency, are transformed from a paste-consistency wet film to a dry coating by exposure to a strong ultraviolet (UV) light source for a predetermined period of time. These photopolymers are further characterized as being imaging quality, or capable of being selectively hardened by light passing through a photographic master, thereby capable of being transformed into a film securely affixed to a substrate at locations where the photographic master allowed the UV light to impinge upon the photopolymer. For example, these photopolymers are available commercially as plating resists and as etch resists for use in the manufacture of PWBs, and also for graphics imaging. Such photopolymers can be applied conventionally by screen printing the wet photopolymer through an image bearing screen stencil to deposit images on the substrates which are then hardened and transformed into permanent images by being subjected to a strong UV light source.
Such screen printed photopolymer images are characterized by large energy expenditure, heated substrates, indistinct boundaries, loss of fidelity, and smeared images.
The images which can be achieved using the disclosed process and apparatus and the same photopolymers are characterized by lines having sharp, distinct boundaries, and exceptional fidelity with film thicknesses up to 0.002 inch (0.05 mm) without any smearing. For example, conventional screen printed PWB resist patterns are practically limited to conductor widths and spacing of 0.010 inch (2.54 mm) minimum, while the same photopolymers can be imaged as disclosed herein to produce line widths and spacing of 0.003 inch (0.076 mm), with a film thickness of 0.00025 inch (0.006 mm). Conventional screen printed half-tone images are limited to a practical upper range of 105 lines, with dot sizes of 20 to 80 percent. The same photopolymer imaged as described herein can be transformed into half-tone graphics of 150 lines, with dot sizes of 5 to 95 percent.
Conventional preparation of photopolymer relief printing plates are described in U.S. Pat. No. 4,070,110, issued Jan. 24, 1978, and in U.S. Pat. No. 4,087,182, issued May 2, 1978. These are contact printing processes as compared to a photo imaging process such as described in U.S. Pat. No. 4,052,603, issued Oct. 4, 1977. All of these involve complex and expensive machinery and could not result in simple single pass lamination and exposure scanning as can the present invention.
U.S. Pat. No. 4,159,176, issued June 26, 1979, discloses a device for aligning a photomask onto a printed circuit board. An exposure frame is used to align and hold the photomask in registration with a printed circuit board which has been coated with a photosensitive material.
U.S. Pat. No. 3,948,657, issued Apr. 6, 1976, discloses a method for adhering a photoconductive layer to an insulating layer, using an adhesive. The insulating layer is brought into contact with the adhesive coated photoconductive layer by means of a squeegee, which can be a roller or blade, for example, a coating knife.
U.S. Pat. No. 4,506,004, issued Mar. 19, 1985, discloses a method for preparing a printed wiring board. A photopolymer is brought into contact with a liquid polymer layer to mate with the printed wiring board by using a knife blade.
U.S. Pat. No. 4,424,089, issued Jan. 3, 1984, discloses a process for applying a paste-consistency photopolymer to a printed wiring board. Photographic film is brought into intimate contact with the photopolymer using a resilient blade.
U.S. Pat. No. 4,260,675, issued Apr. 7, 1981, discloses a method for preparing a printed circuit board solder mask. The exposure assembly, comprised of a coated PWB between two exposure plates, is connected to a vacuum source and is evacuated so that atmospheric pressure holds the plates and PWB together while the assembly is conveyed under a UV lamp.
U.S. Pat. No. 3,837,887, issued Sept. 24, 1974, discloses a process for the preparation of printing plates of photo-sensitive resin. A method is disclosed for applying a sheet smoothly over a doctored surface of photosensitive resin by using a roller to advance the film and smoothly apply it to the photosensitive resin. In one embodiment, the film is applied using the roller and a separate idler roller which urges the sheet against the roller.
U.S. Pat. No. 4,528,261, issued July 9, 1985, discloses another application of a pressure roller to form a laminate having a photohardenable liquid layer.
The present invention provides a simplified and improved process and apparatus over prior art equipment and processes for producing printed elements of high resolution with liquid photopolymers,